Unless you go with shorty’s or actual custom built headers, 99% of all headers will require "massaging" to install (whether it's denting a tube, moving a bracket, etc.). Unless it is for racing purposes, in all reality, there are little differences in overall performance between shorties (Tri-Y), mid-length & long tube headers.....shorties actually perform excellent up to about 5500 rpm and then will have a slight drop until 6k rpm when it begins to really decline.....more than enough for street use and much better than oem manifolds.
Hedmann in the 60's/70's an excellent product and considered one of the top mfg's with Hooker....they were built in Culver City, Ca until a few years ago. Beginning in the late 80's Hedmann quality began declining & today, they are considered an "inexpensive Header", meaning, you get what you pay for. In general, the head plate should be at least 3/8" (1/2" is best), tubing should be at least 16 gauge (14 is best).....otherwise leaks and burn-though are going to be a problem as well as the longetivity of the headers.